Currently, there are mainly two coating methods for liquid crystal alignment film of TFT-LCD: PI (polyimide) roller coating and PI inkjet, and the entire TFT-LCD industry employs both of roller coating and inkjet coating, or one of them.
Referring to FIG. 1, a process for printing technique mainly includes: a polyimide (PI) solution is ejected from a PI nozzle 4 and it drips on a turning metal roller 3, and then a scraper 5 is used to scrape excessive PI solution. The metal roller 3 rotates and is in contact with a plate cylinder 6, and the PI solution is stored on a printing plate 7 which is attached on the plate cylinder 6. Then, the plate cylinder 6 rotates and is in contact with a glass substrate 1 on a printing stage 2, finally the PI solution is printed on the glass substrate 1. The roller coating technique has the following drawbacks: cumbersome equipment, poor mobility, large consumption of consumables (dummy substrates and printing plates), thick film which is difficult to adjust, complicated operations, low PI usage percentage, high operating costs and common deficiencies easily caused by polluted printing plates.
Referring to FIGS. 2 and 3, PI printing is controlled by using deformation of piezoelectric ceramics, so that a PI drip 8 is dripped from a nozzle hole 9 and land on a glass substrate 1 disposed on a printing stage 2. The PI drip 8 is then evenly spread on the glass substrate 1, and an even film is formed by diffusion of the PI drip 8. This technical method has the following drawbacks: large uneven area of thick edges of the film, small nozzle hole which is easy to be clogged, interval difference of the nozzle hole 9 and uneven spreading which will produce deficient products with uneven inner lines and inner surfaces. The most serious deficiency is that, the PI solution is unable to be spread evenly to form a film when a segmental difference of the substrate surface is too large. As a result, product quality is substantially decreased and possible large quantity of rejected products. Furthermore, halo area is also an important factor to affect the printing quality. Therefore, the conventional coating and inkjet techniques need to be improved.